The

The sun has played a dominant role since time immemorial for different natural activities in the universe at large and in the earth in particular for the formation of fossil and renewable energy sources. It will continue to do so until the end of the earth's remaining life, which is predicted to be about 5 x 109 years. Deposited fossil fuels, in the form of coal, that are used through combustion are expected to last for approximately the next 300 years at the most, and from then onward human beings will be left with renewable energy resources only.

The diameter of the sun is R = 1.39 x 106 km. The sun is an internal energy generator and distributor for other planets such as the earth. It is estimated that 90% of the energy is generated in the region between 0 and 0.23R, which contains 40% of the sun's mass. The core temperature varies between 8 x 106 K and 40 x 106 K and the density is estimated at about 100 times that of water. At a distance 0.7R from the center the temperature drops to about 130,000 K where the density is about 70 kg/m3. The space from 0.7R to 1.0R is known as the convective zone with a temperature of about 5000 K and the density is about 10-5 kg/m3.

The observed surface of the sun is composed of irregular convection cells with dimensions of about 1000-3000 km and with a cell life time of a few minutes. Small dark areas on the solar surface are referred to as pores and have the same order of magnitude as the convective cells; larger dark areas are sunspots of various sizes. The outer layer of the convective zone is the photosphere with a density of about 10-4 that of air at sea level. It is essentially opaque as the gases are strongly ionized and able to absorb and emit a continuous spectrum of radiation. The photosphere is the source of most solar radiation. The recessing layer is above the photosphere and is made up of cooler gases several hundred kilometers deep. Surrounding this layer is the chromosphere with a depth of about 10,000 km. It is a gaseous layer with temperatures somewhat higher than that of the photosphere but with lower density. Still further out is the cornea, which is a region of very low density and very high temperature (about 106 K). Solar radiation is the composite result of the abovementioned several layers.

An account of the earth's energy sources and demand cannot be regarded as complete without a discussion of the sun, the solar system, and the place of the earth within this system. In general, the sun supplies the energy absorbed in the short term by the earth's atmosphere and oceans, but in the long term by the lithosphere where the fossil fuels are embedded. Conversion of some of the sun's energy into thermal energy derives the general atmospheric circulation (Becquerel 1839). A small portion of this energy in the atmosphere appears in the form of the kinetic energy of the winds, which in turn drive the ocean circulations. Some of the solar energy is intercepted by plants and is transformed by photosynthesis into biomass. In turn, a large portion of this is ultimately converted into heat energy by chemical oxidation within the bodies of animals and by the decomposition and burning of vegetable matter. On the other hand, a very small proportion of the photosynthetic process produces organic sediments, which may eventually be transformed into fossil fuels. It is estimated that the solar radiation intercepted by the earth in 10 days is equivalent to the heat that would be released by the combustion of all known reserves of fossil fuels on earth.

Until the rise of modern nuclear physics, the source of the sun's energy was not known, but it is now clear that the solar interior is a nuclear furnace that releases energy in much the same way as man-made thermonuclear explosions. It is now obvious through spectroscopic measurements of sunlight reaching the earth from the photosphere layer of the sun that the solar mass is composed predominantly of the two lightest elements, hydrogen, H, which makes up about 70% of the mass, and helium, He, about 27%; and the remaining 3% of solar matter is made up of all the other 90 or so elements (McAlester 1983). The origin of solar radiation received on the earth is the conversion of H into He through solar fusion. Theoretical considerations show that, at the temperatures and pressures of the solar interior, He is steadily being produced from lighter H as four nuclei unite to form one nucleus of helium as presented in Fig. 3.1. During such a conversion, single H nuclei (proton) made unstable by heat and pressure, first combine to form double H nuclei; these then unite with a third H nucleus to form3 He, with a release of electromagnetic energy.

The sun is a big ball of plasma composed primarily of H and He and small amounts of other atoms or elements. Plasma is a state of matter where the electrons are separated from the nuclei because the temperature is so high and accordingly the kinetic energies of nuclei and electrons are also high. Protons are converted into He nuclei plus energy by the process of fusion. As schematically shown in Fig. 3.2 nuclei are composed of nucleons that come in two forms as protons and neutrons with positive and no charges, respectively.

This reaction is extremely exothermal and the free energy per He nuclei is 25.5eV or 1.5 x 108 (kcal/g). The mass of four protons, 4 x 1.00723, is greater than the mass of the produced He nucleus 4.00151 by 0.02741 mass units. This small excess of matter is converted directly to electromagnetic radiation and is

Solar radiation (Energy)

Fig. 3.1 H burning in sun (McAlester 1983)

Solar radiation (Energy)

Neutron

Proton

Fig. 3.1 H burning in sun (McAlester 1983)

Neutron

Proton

Hydrogen

> Helium

Kinetic energy + Gamma rays + Neutrinos

Hydrogen

> Helium

Energy

Fig. 3.2 Proton conversion into He nuclei plus energy the unlimited source of solar energy. The source of almost all renewable energy is the enormous fusion reactor in the sun which converts H into He at the rate of 4 x 106 tonnes per second. The theoretical predictions show that the conversion of four H atoms (i. e., four protons) into He using carbon nuclei as a catalyst will last about 10 years before the H is exhausted. The energy generated in the core of the sun must be transferred toward its surface for radiation into the space. Protons are converted into He nuclei and because the mass of the He nucleus is less than the mass of the four protons, the difference in mass (around 5 x 109 kg/second) is converted into energy, which is transferred to the surface where electromagnetic radiation and some particles are emitted into space; this is known as the solar wind.

It is well known by now that the planets, dust, and gases of the solar system that orbit around the enormous central sun contain 99.9% of the mass of the system and provide the gravitational attraction that holds it together. The average density of the sun is slightly greater than of water at 1.4g/cm3. One of the reasons for sun's low density is that it is composed predominantly of H, which is the lightest element. Its massive interior is made up of matter held in a gaseous state by enormously high temperatures. Consequently, in smaller quantities, gases at such extreme temperatures would rapidly expand and dissipate. The emitted energy of the sun is 3.8 x 1026 W and it arises from the thermonuclear fusion of H into He at temperatures around 1.5 x 106K in the core of the sun, which is given by the following chemical equation (§en 2004) and it is comparable with Fig. 3.2:

In the core of the sun, the dominant element is He (65% by mass) and the H content is reduced to 35% by mass as a direct result of consumption in the fusion reactions. It is estimated that the remaining H in the sun's core is sufficient to maintain the sun at its present luminosity and size for another 4 x 109 years. There is a high-pressure gradient between the core of the sun and its perimeter and this is balanced by the gravitational attraction of the sun's mass. The energy released by the thermonuclear reaction is transported by energetic photons, but, because of the strong adsorption by the peripheral gases, most of these photons do not penetrate the surface. In all regions of the electromagnetic spectrum, the outer layers of the sun continuously lose energy by radiation emission into space in all directions. Consequently, a large temperature gradient exists between the core and the outer parts of the sun.

The sun radiates electromagnetic energy in terms of photons which are light particles. Almost one third of this incident energy on the earth is reflected back, but rest is absorbed and is, eventually, retransmitted to deep space in terms of long-wave infrared radiation. Today, the earth radiates just as much energy as it receives and sits in a stable energy balance at a temperature suitable for life on the earth. In fact, solar radiation is in the form of white light and it spreads over a wider spectrum of wavelengths from the short-wave infrared to ultraviolet. The wavelength distribution is directly dependent on the temperature of the sun's surface.

The total power that is incident on the earth's surface from the sun every year is 1.73 x 1014 kW and this is equivalent to 1.5 x 1018 kWh annually, which is equiv alent to 1.9 x 1014 coal equivalent tons (cet). Compared to the annual world consumption of almost 1010cet, this is a very huge and unappreciable amount. It is approximately 10,000 times greater than that which is consumed on the earth annually. In engineering terms, this energy is considered to be uniformly spread all over the world's surface and, hence, the amount that falls on one square meter at noon time is about 1 kW in the tropical regions. The amount of solar power available per unit area is known as irradiance or radiant-flux density. This solar power density varies with latitude, elevation, and season of the year in addition to time in a particular day (see Sect. 3.7). Most of the developing countries lie within the tropical belt of the world where there are high solar power densities and, consequently, they want to exploit this source in the most beneficial ways. On the other hand, about 80% of the world's population lives between latitudes 35° N and 35° S. These regions receive the sun's radiation for almost 3000-4000 h/year. In solar power density terms, this is equivalent to around 2000kWh/year, which is 0.25 cet/year. Additionally, in these low latitude regions, seasonal sunlight hour changes are not significant. This means that these areas receive the sun's radiation almost uniformly throughout the whole year. Apart from the solar radiation, the sunlight also carries energy. It is possible to split the light into three overlapping groups:

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.